Add async OHTTP key fetch helper for Python

Introduce `fetch_ohttp_keys_async` in the Python bindings and update
integration tests to use it instead of `TestServices.fetch_ohttp_keys()`.
This keeps key retrieval inside Python HTTP code and aligns test flow
with the async sender/receiver path.

The helper fetches `/.well-known/ohttp-gateway` through the configured
relay proxy, validates TLS with the provided DER certificate and decodes the response into `OhttpKeys`.

Author: Mshehu5

payjoin-ffi/python/src/payjoin/http.py

@@ -0,0 +1,28 @@
+import ssl
+from urllib.parse import urljoin
+
+import httpx
+
+from .payjoin import OhttpKeys
+
+
+async def fetch_ohttp_keys(
+    ohttp_relay_url: str,
+    directory_url: str,
+    certificate: bytes,
+) -> OhttpKeys:
+    keys_url = urljoin(directory_url, "/.well-known/ohttp-gateway")
+    ssl_context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
+    ssl_context.check_hostname = True
+    ssl_context.verify_mode = ssl.CERT_REQUIRED
+    pem_certificate = ssl.DER_cert_to_PEM_cert(certificate)
+    ssl_context.load_verify_locations(cadata=pem_certificate)
+    async with httpx.AsyncClient(
+        proxy=ohttp_relay_url, verify=ssl_context, timeout=30.0
+    ) as client:
+        response = await client.get(
+            keys_url,
+            headers={"Accept": "application/ohttp-keys"},
+        )
+        response.raise_for_status()
+        return OhttpKeys.decode(response.content)

payjoin-ffi/python/test/test_payjoin_integration_test.py

@@ -4,6 +4,7 @@
 import json
 
 from payjoin import *
+from payjoin.http import fetch_ohttp_keys
 from typing import Optional
 import unittest
 
@@ -98,7 +99,8 @@ async def test_ffi_validation(self):
         services = TestServices.initialize()
         services.wait_for_services_ready()
         directory = services.directory_url()
-        ohttp_keys = services.fetch_ohttp_keys()
+        ohttp_relay = services.ohttp_relay_url()
+        ohttp_keys = await fetch_ohttp_keys(ohttp_relay, directory, services.cert())
         recv_persister = InMemoryReceiverSessionEventLog(999)
         pj_uri = self.create_receiver_context(
             receiver_address, directory, ohttp_keys, recv_persister
@@ -261,8 +263,8 @@ async def test_integration_v2_to_v2(self):
 
             services.wait_for_services_ready()
             directory = services.directory_url()
-            ohttp_keys = services.fetch_ohttp_keys()
             ohttp_relay = services.ohttp_relay_url()
+            ohttp_keys = await fetch_ohttp_keys(ohttp_relay, directory, services.cert())
             agent = httpx.AsyncClient()
 
             # **********************